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The opisthion is the midpoint on the posterior margin of the foramen magnum. The basion is located at the midpoint on the anterior margin of the foramen magnum. The alar ligament , which is attached on each side to the tubercle of occipital condyle , divides the foramen magnum into an anterior smaller compartment and a posterior larger compartment.
The basion is the most anterior point of the opening and the opisthion is the point on the opposite posterior part. The basion lines up with the dens. Foramen magnum
On its lower surface, about 1 cm. in front of the foramen magnum, is the pharyngeal tubercle which gives attachment to the fibrous raphe of the pharynx.. On either side of the middle line the longus capitis and rectus capitis anterior are inserted, and immediately in front of the foramen magnum the anterior atlantooccipital membrane is attached.
The external occipital crest is part of the external surface of the squamous part of the occipital bone.It is a ridge along the midline, beginning at the external occipital protuberance and descending to the foramen magnum, that gives attachment to the nuchal ligament. [1]
The Powers ratio was formerly used, which was the tip of the basion to the spinolaminar line, divided by the distance from the tip of the opisthion to the midpoint of the posterior aspect of the anterior arch of C1. It is no longer recommended due to low sensitivity and difficulty identifying landmarks.
In neurology, the Chiari malformation (/ k i ˈ ɑː r i / kee-AR-ee; CM) is a structural defect in the cerebellum, characterized by a downward displacement of one or both cerebellar tonsils through the foramen magnum (the opening at the base of the skull).
Near the middle of the squamous part of occipital bone is the external occipital protuberance, the highest point of which is referred to as the inion.The inion is the most prominent projection of the protuberance which is located at the posterioinferior (rear lower) part of the human skull.
Cephalometric analysis depends on cephalometric radiography to study relationships between bony and soft tissue landmarks and can be used to diagnose facial growth abnormalities prior to treatment, in the middle of treatment to evaluate progress, or at the conclusion of treatment to ascertain that the goals of treatment have been met. [5]